There are 2 ways to Unsubscribe a contact from a group.
One by one
Select the crossed sign ( Ø ) that appears when hovering over or after checking a particular contact.
At the bottom of your list of contacts, click on select all.
From the two new commands that appear, click on unsubscribe.
You can also select a subset of contacts (by clicking on them one by one) and then selecting "unsubscribe selected contacts".